This invention relates to a process for producing highly pure isophthalic acid which is useful as an intermediate for preparing polymers, such as an unsaturated polyester resin, alkyd resin and thermal resistant polyamide, etc., and particularly relates to a process for producing highly pure isophthalic acid which comprises hydrogenating the impurities contained in crude isophthalic acid which has been obtained by liquid phase-oxidizing an m-alkyl benzene.
Usually, a benzene dicarboxylic acid is produced by oxidizing the corresponding dialkyl benzene. For example, isophthalic acid is produced by liquid-phase oxidizing an m-alkyl benzene.
British Patent No. 1,555,246 discloses a process for producing isophthalic acid comprising liquid phase-oxidizing m-xylene in an aliphatic carboxylic acid, such as acetic acid in the presence of a catalyst comprising cobalt, manganese and bromine. In addition, U.S. Pat. No. 3,974,214 and Belgian Patent No. 871044 disclose a process of producing isophthalic acid comprising liquid phase-oxidizing m-xylene in the presence of the above catalyst and an aldehyde or ketone as an oxidation promoter.
British Patent No. 1,577,544 discloses liquid phase oxidation of m-xylene in a solvent other than acetic acid, or a process for producing isophthalic acid which comprises liquid phase-oxidizing m-xylene in an aqueous solution of benzoic acid in the presence of a catalyst comprising cobalt, manganese and bromine. In addition, U.S. Pat. No. 4,259,522 discloses a process for producing isophthalic acid which comprises oxidizing m-toluic acid in an aqueous solution in the presence of a catalyst comprising cobalt and manganese.
With the recent advances in industrial technology, the requirements to be satisfied by polymer products as engineering materials have become increasingly stringent and there is a demand for isophthalic acid that has sufficient purity and whiteness to justify use as a starting material for the manufacture of polymers. However, the isophthalic acid which has been produced according to the prior methods contains a large amount of impurities, such as 3-carboxy benzaldehyde (hereinunder referred to as 3CBA), etc. The polymers derived from the isophthalic acid containing a large amount of impurities have poor color. Such polymers were not suitable as engineering materials. Therefore, it was necessary to purify the isophthalic acid for preparing the engineering polymer materials. British Patent Nos. 1,152,575 and 1,152,576 disclose a process for purifying crude isophthalic acid which comprises hydrogenating the impurities of the crude isophthalic acid in the presence of a palladium catalyst at a high temperature in a state of an aqueous solution. That is, water is used as a reaction medium in the inventions given in British Patent Nos. 1,152,575 and 1,152,576. However, water which is an inert solvent in the references is not necessarily a good solvent for organic substances. Therefore, when crude isophthalic acid is contacted with a noble metal catalyst in water, impurities, such as toluic acid, which is a reduction product of 3CBA in the crude isophthalic acid and the like are likely to be incorporated into the purified isophthalic acid and as a result the color of the polymers derived from the isophthalic acid could not sufficiently be improved.
British Patent No. 1,152,575 describes improvement in crystallizing step in order to prevent the incorporation of impurities, e.g. toluic acid into the isophthalic acid. Such a process makes the operation and the apparatus therefor complicated, and as a result the process is not economical. In addition, toluic acid and isophthalic acid are incorporated into the mother liquor from which isophthalic acid crystal has been separated, and are discharged with the mother liquor. This not only lowers the yield of isophthalic acid, but also the toluic acid and the isophthalic contained in the exhaust solution causes water pollution. Therefore, it becomes necessary to treat the exhaust solution in order to prevent water pollution.